1. Field of the Invention
The present inventions relate to thin film solar cell fabrication methods and structures.
2. Description of the Related Art
Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce the cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA chalcopyrite compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) or a grid pattern may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula for a CIGS(S) layer is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
One way of reducing the cost of thin film photovoltaics is to process thin film CIGS(S) type solar cells on flexible metallic foils so that the depositions of multiple films or layers constituting the solar cell structure, such as the contact layer, the CIGS(S) absorber film, the transparent layer and the metallic grids, may all be performed over the flexible foil substrate in a roll-to-roll fashion. This way a long (such as 500-5000 meters long) foil substrate may be processed in relatively compact process tools to form a roll of solar cells, which may then be cut and used for module fabrication. Choice of the substrate material is very important for thin film solar cells since the layers in these device structures are only 1-5 micrometers (μm) thick and they get affected by the nature of the substrate during and after processing. For example, the typical thicknesses of the contact layers, the CIGS absorber layers and transparent layers are 0.3-1μ, 1-3μ and 0.1-0.5μ, respectively.
Stainless steel foils are widely used flexible metallic substrates to manufacture CIGS solar cells. However, one problem associated with the use of stainless steel substrates is the surface roughness of the foil and the defects associated with the surface roughness. Topographical defects such as protrusions, gouges and spikes often appear on stainless steel foils and they can originate from different sources such as foreign particle (such as metal particle or ceramic particle) inclusions in the steel foil, defects in the steel and rolling defects and particles which may be introduced on the steel foil surface during the rolling process for foil formation. Depending on their size, such defects not only reduce the efficiency of the solar cells manufactured on such foil surfaces but may also cause complete electrical shunts which are detrimental to solar cell performance.
FIG. 2 illustrates a solar cell device structure 50 built on a defective area of a stainless steel foil substrate 52 to exemplify a plausible mechanism that forms shunts within the solar cell device structure. The surface of stainless steel foil substrate includes exemplary protrusions 54 or spikes which may have a height in the range of 100-2000 nm, or even higher. A contact layer or conductive layer 56 is typically a 200-1000 nm thick film of a refractory metal such as Mo, typically deposited by a physical vapor deposition technique such as sputtering. Therefore, when deposited over the rough surface of the substrate 52, the contact layer 56 may have discontinuities or defective portions where the underlying substrate is exposed, especially over the locations of the protrusions 54. When an absorber film 58 is formed over the contact layer 56, the substrate 52 would be exposed to the absorber film 58 and to the reactive atmosphere that is present during the growth of the absorber film 58 at the locations of the discontinuities. As a result, chemical interaction may take place between the substrate 52 and the constituents of the absorber film 58 at these locations. For example, for stainless steel substrates, iron in the stainless steel may react with the Se in the CIGS film growth environment forming iron selenides in the defect regions. Also iron atoms diffusing from the substrate 52 into the absorber 58 through the discontinuities in the contact layer 56 may poison the regions of the absorber 58 directly above the discontinuities. When the solar cells are completed by deposition of the transparent layer 60, these poisoned regions cause excessive current leakage or shunts reducing the efficiency of the devices. It should be noted that excessive iron in CIGS material lowers its resistivity and degrades its photovoltaic performance. In an alternate mechanism, the regions where the contact layer 56 is discontinuous may not be coated effectively by the absorber film 58, giving rise to pinholes 62 that may cause shunts, which are killer defects for solar cells. As can be seen in FIG. 2, the transparent layer 60 is in direct contact with the substrate 52, through the protrusions 54 at the locations of the pinholes 62.
Various surface planarization methods, including electropolishing, do not effectively reduce the surface roughness of such stainless steel foils because etchants also etch the surface portions around the peaks and deepen such portions. As a result of this, the spikes or the peaks tend to stay and in most cases new ones are created as more defects such as inclusions hidden in the foil body get exposed through the etched surface.
From the foregoing, the much needed manufacturing process yield improvements require identification and development of metallic foil substrates and the processes that are better suited for large volume roll-to-roll processing and manufacturing of CIGS solar cell and modules.